Structural health monitoring (SHM) corresponds to methodologies for determining the health of a structure based on readings from one or more sensors embedded into the structure and monitored over time. Exemplary monitored structures may include buildings, bridges, dams, aircraft, and any other structures the physical integrity of which may be of significance to any interested parties.
Structural health monitoring may be conducted passively or actively. Passive SHM may be conducted by monitoring a number of parameters including, as non-limiting examples, loading stress, environmental action, performance indicators, and acoustic emissions from cracks or other sources. Inferences may then be made of the state of structural health from a structural model based on the monitored parameters. Active SHM may be conducted by performing proactive interrogation of the structure. The state of structural health may then be determined from an evaluation of detected parameters.
Both approaches perform a diagnosis of structural health and safety, to be followed by a prognosis of the remaining life of the structure. Passive SHM uses passive sensors that only “listen” but do not interact with the structure. Therefore, they do not provide direct measurement of damage presence and intensity. Active SHM uses active sensors that interact with the structure and thus determine the presence or absence of damage. Exemplary methods used with active SHM resemble methods of nondestructive evaluation (NDE) corresponding to ultrasonic testing and eddy current testing, except that they are used with embedded sensors. Hence, active SHM could be viewed as a method of embedded nondestructive evaluation.
One widely used active structural health monitoring method employs piezoelectric wafer active sensors (PWAS), which send and receive Lamb waves and may be used to determine the presence of cracks, delaminations, disbonds, and corrosion. Due to its similarities to nondestructive evaluation ultrasonics, this approach is also known as embedded ultrasonics. Although PWAS are small, unobtrusive, and inexpensive, the laboratory measurement equipment used, for example as a proof-of-concept demonstration of the present technology, is bulky, heavy, and relatively expensive. Such laboratory equipment cannot be easily transported into the field for on-site structural health monitoring. Therefore, such equipment is less desirable for large-scale deployment of the presently disclosed electromechanical (E/M) impedance technology for SHM applications.
Several investigators have explored means of reducing the size of the impedance analyzer, to make it more compact, and even field-portable. Alternative ways of measuring the E/M impedance, which are different from those used by the impedance analyzer, have also been considered. One approach as illustrated in FIG. 1 by Pardo de Vera and Guemes (1997), at the Polytechnic University of Madrid, Spain, employed an E/M impedance technique to detect damage in a composite specimen 12 by way of a piezoelectric sensor 14 using a simplified impedance measuring method corresponding to the use of an RC-bridge 10 instead of a laboratory impedance analyzer. Disadvantages associated with using the RC-bridge 10 included the fact that additional instrumentation and processing needed to be used to separate the signal into its real (in phase) and imaginary (out of phase) parts, and precise bridge balance needed to be initially attained in order to prevent the excitation signal from leaking into the output and masking the sensing signals.
In addition to the above, Peairs, Park and Inman (2002) suggested a method of generating impedance measurements utilizing an FFT analyzer 20 and small current measuring circuit 22 as illustrated in FIG. 2. The approximated impedance (ZPZT) is: ZPZT=Vi/I, where I is the current through the sensing resistor (RS). Disadvantages associated with this method include the fact that Vi is not the exact voltage across the PZT but rather is an approximation, so that there may be an unacceptable error for measured ZPZT. In addition, the error will increase with frequency so that, at high frequencies of approximately 100 kHz for a gain of 20 dB with a standard 741-type integrated circuit operational amplifier, the amplifier becomes ineffective due to roll-off of the output signal. Finally, a large laboratory instrument-FFT analyzer is still needed.
While various implementations of nondestructive evaluation (NDE) devices have been developed, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.